The former Notts County and Yeovil Town keeper, 30, suffered ligament damage that will keep him on the sidelines for at least a month.
"That's the very least amount of time Steve's going to be out and of course it's disappointing news that we're going to be without him," boss Steve Tilson told the club's official website.
"We obviously knew that he was going to be out for a while and it's a little better than we first thought because the ankle isn't broken but it's still frustrating.
"He'd done well for us, especially on Saturday, and it's a blow that we're going to be without him for a minimum of a month."